1. Field of the Invention
The present invention relates to a method and system for generating electrical energy within a vehicle tyre. More specifically, electrical power is generated by using piezoelectric technology to convert mechanical strain, due to tyre flexure during rolling, to electric charge, that is stored in an energy storage device, and is thus made available to an electronic device disposed within the tyre. The present invention also relates to a system for monitoring operating parameters of a tyre.
2. Description of the Related Art
The incorporation of electronic devices within pneumatic tyres is taking a greater importance in order to increase safety of vehicles. Tyre electronics may include sensors and other components suitable for obtaining information regarding various physical parameters of a tyre, such as for example temperature, pressure, number of tyre revolutions, vehicle speed. Such information may become useful in tyre monitoring and/or warning systems. Furthermore, active control systems of the vehicle may be based on information sent from sensor devices included within the tyres. Typically, wireless transmission is employed in order to send the tyre performance information outside the tyre, to a receiver disposed on the vehicle, so that such electronic devices disposed within the tyre typically include a transmitter associated to an antenna. A microprocessor is also typically employed, in order to collect and process the signals coming from the performance sensors, before transmission.
Such integrated tyre electronics have conventionally been powered by a variety of techniques and different power generation systems.
A typical solution for powering tyre electronics systems is the use of a non-rechargeable battery, which may cause inconveniences to a tyre user since proper electronics system operation is dependent on periodic battery replacement. As a matter of fact, batteries tend to deplete their energy storage quite rapidly when powering electronic applications characterized by complex levels of functionality. Furthermore, conventional batteries typically contain heavy metals that are not environmentally friendly and which present disposal concerns, especially when employed in numerous quantity. Moreover, performances of conventional batteries are often influenced by temperature: in particular, the functioning of such batters is not reliable at low temperatures.
Another known method for powering tyre monitoring systems is a coupling of radio-frequency (RF) power between an antenna disposed on the vehicle in close proximity with an antenna included within the electronic device disposed in the tyre. This typically requires antennas disposed in vehicle portions frequently exposed to damage from road hazards, and thus may not be a desirable solution for powering tyre electronic applications.
The use of piezoelectric elements has also been proposed for powering tyre monitoring systems. Piezoelectricity is a property of certain materials, such as quartz, Rochelle salt, and certain solid-solution ceramic materials such as lead zirconate-titanate (PZT), of generating electricity when mechanically stressed.
For example, PCT patent application WO 01/80327 A1 discloses a system for generating electrical energy in a vehicle tyre, comprising at least one elongate piezoelectric element which extends in a longitudinal direction along at least a portion of the tyre. The elongate piezoelectric element preferably comprises a coaxial cable extending along a straight or undulated path of the tyre circumference.
PCT patent application WO 03/095244 A1 discloses a system for generating electric power from a rotating tyre's mechanical energy that has a piezoelectric structure and an energy storage device. The structure comprises a plurality of piezoelectric fibers embedded in a generally unidirectional fashion in an epoxy matrix. The structure is mounted on a support substrate for uniformly distributing mechanical strain in the piezoelectric structure. The structure is mounted within a tyre for generating electric charge as the wheel moves along a ground surface.
U.S. Pat. No. 4,510,484 discloses a device provided for sensing the condition of a pneumatic tyre mounted on a tyre rim and subject to normal vibrations. The device comprises a housing, a band for mounting the housing to the tyre rim, a sensor for monitoring the condition within the tyre, circuitry operatively connected to the sensor for generating radio signals indicative of the tyre condition, power supply operatively connected to the circuitry and a receiver for receiving the radio signals. The power supply includes a radially extending piezoelectric reed having a base portion and an end portion. The base portion is elastomerically bonded to the housing. A tuning mass member is mounted to the end portion and is configured for mating abutment against stop members which limit the flexure stroke of the piezoelectric reed and inhibit the compound bending of the reed. The tuning mass member is sized relative to the piezoelectric reed to obtain a natural resonant frequency of vibration of the power supply of approximately 60 Hz, corresponding to common wheel vibrations which occur during vehicle operations. In operation, centrifugal forces operate to urge the tuning mass member away from the radiating center of the rotating wheel. Such forces tend to align the plane defined by the piezoelectric reed element with a radiating center line. In the event the reed element is not aligned at a rest equilibrium state with a radiating center line, centrifugal forces cause the reed element to bend into such an alignment and may urge the tuning mass member into continued engagement with an adjacent stop member. Such continued engagement would operate to reduce the vibration of the reed element and accordingly reduce the ability of the power supply to power the radio circuit. When the reed element is properly aligned along a radiating center line, the power supply may enjoy a maximum vibrational stroke during operation with optimum ability to power the radio circuit.
U.S. Pat. No. 6,438,193 discloses a self-powered revolution counter of a tyre, comprising a mechanical-electrical energy converter and a revolution counting circuit. One piezoelectric crystal element acts both as energy converter and as revolution sensor. The piezo element is attached to or embedded within the inner wall of the tyre, under the tread or the sidewall, in a way which causes it to flex with the tyre each time the circumferential sector of the tyre containing the piezo element is compressed against the road or other vehicle-supporting surface. A positive pulse is generated when the piezo element is flexed. When straightened again, the piezo element produces damped oscillating positive/negative signal at a significantly lower peak level than the positive pulse. The damped oscillations are determined by physical characteristics of the piezo element (mass, compliance). A typical measured oscillation frequency is on the order of 100 Hz. According to the authors, these oscillations are beneficial for enemy conversion. A prepared embodiment of the piezo element disclosed in the '193 patent is a circular unimorph having two circular plates bonded together and a piezo crystal plate in the center. According to the authors, the stress distribution is more uniform in this configuration than that obtained with a bimorph implemented in a typical cantilever mounting. The authors also disclose that an open voltage of 34 V was obtained with a large deflection in a tested cantilever-mounted bimorph piezo element. However, the Applicant notes that details related to the structure of the tested cantilever-mounted bimorph piezo element and to its mounting within the tyre are not disclosed in the '193 patent.